ARM64: dts: meson-axg: enable ethernet for A113D S400 board

This is tested in the S400 dev board which use a RTL8211F PHY,
and the pins connect to the 'eth_rgmii_y_pins' group.

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>